My research has dealt with the dynamics of reaction and photodissociation with the goal of understanding the essential features of chemistry in both gases and liquids. The unifying theme of this research is connecting chemical reaction dynamics occurring in gases to those in liquids. We use both high resolution lasers and ultrafast lasers, and the key to all of our experiments is preparing molecules in vibrationally excited states and spectroscopically monitoring their subsequent behavior.
Vibrational excitation is crucial in many chemical reactions because motion of the atoms relative to each other carries the system through the transition state that lies atop the barrier to reaction. Because laser excitation is a particularly attractive means of preparing molecules in specific internal states, our strategy is to excite molecules with a laser pulse and use time-resolved spectroscopy to follow their subsequent behavior. We use high resolution lasers in some experiments and ultrafast lasers, which produce pulses of less than 100 fs duration, in others. The two approaches often provide complementary information and allow us to study some of the same reactions in isolated molecules and in liquids. In the former case, laser excitation prepares a molecular eigenstate that does not evolve in time and whose properties we exploit to control the course of a chemical reaction. In the later case, the short laser pulse prepares a state that does evolve in time and that we intercept at different points in its evolution. This good time resolution allows us to observe processes that occur during a time that is comparable to the interval between the interactions in solution.
We use a variety of excitation and detection techniques, such as resonant multiphoton ionization with ion imaging in molecular beams and time-resolved transient absorption or non-linear spectroscopy in liquids. The molecular beam experiments provide an extremely detailed view of the chemical dynamics of isolated, well-characterized molecules. We have exploited our understanding of the behavior of vibrationally excited molecules to control the course of a chemical reaction, and we have used laser excitation to cleave a particular bond selectively in both photodissociation and bimolecular reaction. The ultrafast laser techniques now allow us to follow the flow of energy within a molecule directly and to study vibrationally driven reactions in liquids. Discovering the controlling aspects of chemical reactions at a fundamental level is the focus of our research.